The innate immune response alerts the host to infection before an adaptive immune response has a chance to develop. Viral nucleic acids introduced into cells or synthesized during infection trigger cytosolic sensors of innate immunity to produce antiviral cytokines, such as type I interferons (IFN). Some viruses evade immune sensing. The increase in viremia during acute stages of infection are associated with widespread activation of DCs and activation of IFN, although HIV infection of target cells such as T cells and macrophages does not induce intrinsic antiviral IFN responses. This application focuses on intracellular innate immunity in target cells and aims to understand how HIV avoids innate immune activation. Prior to work conducted by the Principal Investigator (PI), intracellular innate immune recognition of retroviruses especially HIV has been a key information gap in our existing knowledge regarding innate immune recognition of viruses. We recently discovered that the cytosolic exonuclease TREX1 suppressed the IFN response triggered by HIV DNA. In Trex1-/- mouse cells and human CD4+ T cells and macrophages in which TREX1 was inhibited by RNAi, HIV DNA accumulated in the cytosol and HIV infection induced IFN production that inhibited HIV replication and spreading. Our study suggested that cytosolic HIV DNA is a key pathogen-associated molecular pattern (PAMP) that can be detected by innate immunity and activate IFN and pro-inflammatory cytokine production, and that HIV exploits host factors (such as TREX1) to subvert innate immune detection. We also provide extensive preliminary data that uncover a cytosolic DNA detection pathway involved in the recognition of HIV DNA, and a novel mechanism by which TREX1 inhibits IFN induction. The specific aims of this proposal are 1) To define the role(s) of an innate immune DNA sensing pathway in the recognition of cytosolic HIV DNA. 2) To characterize how TREX1 inhibits the innate immune response to HIV. 3) To characterize how the opposing dynamic between HIV innate immune detection and HIV innate immune evasion contributes to the establishment of infection by HIV. Our proposed study will elucidate the molecular mechanisms of how HIV subvert innate immune responses, and lay the foundation for harnessing intracellular innate immunity as a novel avenue of therapeutics.